Paweł Zięba

The kinetics of the discontinuous precipitation and dissolution in Mg-rich Al alloys

The morphology and growth kinetics of the discontinuous precipitation and dissolution reactions in supersaturated Mg-Al solid solutions containing 7.3, 9.1 and 10.9 at % Al have been investigated by optical and scanning electron microscopy and X-ray measurements. The volume fraction of regions transformed by the discontinuous precipitation reaction, the reaction front velocity, the interlamellar spacing and the average composition of the solute-depleted lamellae were determined as a function of the temperature. For the first time, the kinetics of the discontinuous dissolution reaction has been studied in the Mg-Al system. It has been shown that the transport of the solute atoms during both reactions is governed by grain boundary diffusion.

Phase transitions induced by severe plastic deformation: steady-state and equifinality

Abstract During severe plastic deformation (SPD), a steady-state is usually reached after a certain value of strain (i. e. number of passes during equal-channel pressing or number of rotations during high pressure torsion). The structure and properties of a material in a steady state (including composition of phases) do not depend on those in the starting state before SPD. In other words they are equifinal, and the production of lattice defects is in dynamic equilibrium with defect elimination. Moreover, the SPD-treatment at ambient temperature TSPD = 300 K is frequently equivalent to the heat treatment at a certain elevated temperature Teff > 300 K. For example, the composition of phases in Cu–Ni, Co–Cu and Nd–Fe–B-based alloys after high pressure torsion corresponds to the states at 200, 890 and 1 170 °C, respectively, and is rather insensitive to the high pressure torsion rate (between 0.2 and 2 rpm) and pressure (between 3 and 8 GPa).

Heat effect of grain boundary wetting in Al-Mg alloys

Grain boundary wetting transitions were previously observed in the Al–Mg system. The melting of as-cast Al–5 wt% Mg and Al–10 wt% Mg alloys was studied by the differential scanning calorimetry. The asymmetric shape of the melting curve permitted the observation of the thermal effect of grain boundary wetting. The difference in the shape of the melting curve for the two studied alloys is explained by the different temperature dependence of the fraction of completely wetted grain boundaries.

Transformation of Hume-Rothery phases under the action of high pressure torsion

It has been revealed experimentally that high-pressure torsion induces phase transformations of certain Hume-Rothery phases (electron compounds) to others. High-pressure torsion induces the ξ → δ + ɛ reaction in copper-tin alloys with the appearance of the δ + ɛ phase mixture as after long-term annealing in the temperature range Teff = 350–589°C. The mass transfer rate driven by high-pressure torsion is 14–18 orders of magnitude higher than the rate of conventional thermal diffusion at the processing temperature THPT. This phenomenon can be explained by an increased concentration of defects (in particular, vacancies) in the steady state under high-pressure torsion, which is equivalent to an increase in the temperature.

A new mode of the discontinuous dissolution reaction in Mg-10 wt.% Al alloy

The occurrence of discontinuous precipitation (DP) and dissolution reactions has been investigated in Mg-10 wt.% Al alloy. The application of a special cyclic heat treatment allowed to observe a new mode of the discontinuous dissolution reaction. The reaction starts preferably and occurs faster at the original locations of the grain boundaries, which is in contrast to the commonly observed behaviour where the reaction occurs, in the early stage, primarily at the reaction front of the former DP colonies. The proposed explanation is attributed to a characteristic movement of the reaction front of the DP colony away from and back to the original location of the grain boundary during subsequent cycles of ageing and dissolution.

Influence of phosphorous content on microstructure development at the Ni-P Plating/SAC interface

Studies of the commonly used Ni-P surface finish of 4.3 and 11.6 wt. % of P content electroless plated on nickel substrates followed by their reaction with SAC305 solder were performed. It was demonstrated that the Ni-4.3P plating was crystalline, while the Ni-11.6P was mostly amorphous. The transformation of the Ni-P into Ni3P phase took place at 672 K and 605 K for low and high P amount, respectively. The activation energy (Ea) of the crystallization processes in the Ni-P plating was lower for the Ni-11.6P plating. Interaction of SAC305 solder with both types of the inspected plating showed the creation of (Cu,Ni)6Sn5 phase in the form of thin layer and large scallops, while for Ni-11.6P/SAC305 interface also (Ni,Cu)3Sn4 phase. The thickness of these phases was larger in the case of low phosphorous containing plating. The Ni-11.6P plating after the reaction with SAC305 totally transformed into Ni12P5, while the enrichment in P up to 10.5 wt. % occurred in the Ni-4.3P which did not lead to the appearance of any NixPy type phases. After the reaction of plating with solder the Ni2SnP phase was not identified. This was related to the absence of spalling phenomenon of the intermetallics into solder.

Diffusion and phase transitions accelerated by severe plastic deformation

Severe plastic deformation (SPD) can lead to the phase transformations in the materials. Even the SPD-treatment at ambient temperature TSPD = 300 K is frequently equivalent to the heat treatment at a certain elevated temperature (effective temperature) Teff > 300 K. However, if the real annealing at effective temperature leads to the grain growth, SPD leads to strong grain refinement. SPD also accelerates the mass transfer in the materials. In this review the methods of determination for effective temperature after high-pressure torsion of metallic alloys are discussed as well as SPD-driven acceleration of diffusion.

Diffusional penetration during diffusion-induced grain-boundary migration process in an Al-Zn couple

The existence of diffusion-induced grain-boundary migration (DIGM) has been re-examined by electron probe micro analysis and analytical electron microscopy in the aluminium substrate of Al-Zn diffusion couple annealed in the temperature range 395–535 K. The investigation revealed two basic kinds of DIGM: laminar and turbulent. The laminar kind occurs over the whole temperature range and is characterized by a small migration distance and large migration depth. The zinc enrichment at a sample surface is 4.0–5.0 wt% and gradually decreases with increasing depth. The turbulent kind is limited to annealing temperatures above 450 K. In this case, the width of the alloyed zone is much greater, close to the surface of sample and then dramatically decreases, showing a behaviour similar to the laminar morphology. The zinc content at the surface of sample is 8.0–9.0 wt%. The diffusivities of DIGM calculated based on Cahns equation agree well with the values of stationary grain boundary in diluted AlZn alloys. Evidence for the existence of DIGM was the asymmetry of the zinc profile with regard to the final position of the boundary. Microanalytical scan across the alloyed zone showed an abrupt change of the zinc concentration at the moving boundary. This suggests that the role of volume diffusion during DIGM is not so important and a considerable chemical contribution to the total driving force should exist.

The mechanism of continuous dissolution of the lamellar structure in aged Al 22 at% Zn alloy

The mechanism of continuous dissolution was studied using transmission electron microscopy, X-ray diffraction and hardness measurements. The metastable α′ phase was identified in the first stage of dissolution nucleating at the α-η boundaries. Interface dislocations at the α-α′ boundaries forming a perpendicular net in the 〈110〉 directions were identified as of screw character and of Burgers vector b=1/2a 〈110〉. During dissolution a change of shape of α′ precipitates occurs leading to their fragmentation and to a change of the direction of boundaries into 〈110〉 directions.

Wetting of grain boundaries by the second solid phase in Al-based alloys

This work presents data on changes in the structure and properties of aluminum-based alloys subjected to severe plastic deformation by high-pressure torsion. Microstructure and the composition and change in temperature for phases are studied for double (Al-Zn, Al-Mg) and ternary (Al-Mg-Zn) alloys. Tie-lines of the liquid phase wetting of grain boundaries and grain boundary solvus lines are constructed on phase diagrams of our Al-Zn and Al-Mg systems. The shifting of the phase composition in the bulk of aluminum grains from the Al + τ region to the Al + η region of the Al-Mg-Zn phase diagram with an increase in the specific area of grain boundaries is described.